Kavli Affiliate: Ke Wang
| First 5 Authors: Ke Wang, Yueluo Wang, Fengwei Xu, ,
| Summary:
The initial conditions are critical for understanding high-mass star
formation, but are not well observed. Built on our previous characterization of
a Galaxy-wide sample of 463 candidate high-mass starless clumps (HMSCs), here
we investigate the dynamical state of a representative subsample of 44 HMSCs
(radii 0.13-1.12 pc) using GBT NH3 (1,1) and (2,2) data from the Radio Ammonia
Mid-Plane Survey (RAMPS) pilot data release. By fitting the two NH3 lines
simultaneously, we obtain velocity dispersion, gas kinetic temperature, NH3
column density and abundance, Mach number, and virial parameter. Thermodynamic
analysis reveals that most HMSCs have Mach number $<$5, inconsistent to what
have been considered in theoretical models. All but one (43/44) of the HMSCs
are gravitationally bound with virial parameter $alpha_{mathrm{vir}} < 2$.
Either these massive clumps are in collapsing or magnetic field strengths of
0.10-2.65 mG (average 0.51 mG) would be needed to support them against
collapsing. The estimated B-field strength correlates tightly with density,
$B_{rm est}/{rm mG}=0.269,(n_{rm H_2}/10^4,{rm cm^{-3}})^{0.61}$, with a
similar power-law index as found in observations, but a factor of 4.6 higher in
strength. For the first time, the initial dynamical state of high-mass
formation regions has been statistically constrained to be sub-virial, in
contradictory to theoretical models in virial equilibrium, and in agreement
with the lack of observed massive starless cores. The findings urge future
observations to quantify the magnetic field support in the prestellar stage of
massive clumps, which are rarely explored so far, towards a full understanding
of the physical conditions that initiate massive star formation.
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